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Abstract

Here, we present a systematic study about the effect of the pore length and its diameter
on the specular reflection in nanoporous anodic alumina. As we demonstrate, the specular
reflection can be controlled at will by structural tuning (i.e., by designing the
pore geometry). This makes it possible to produce a wide range of Fabry-Pérot interferometers
based on nanoporous anodic alumina, which are envisaged for developing smart and accurate
optical sensors in such research fields as biotechnology and medicine. Additionally,
to systematize the responsiveness to external changes in optical sensors based on
nanoporous anodic alumina, we put forward a barcode system based on the oscillations
in the specular reflection.

Background

Specular reflection (Rspecular) is the optical property defined as the mirror-like reflection of light/photons from
a surface, in which light/photons from a given incoming direction are reflected into
a single outgoing direction. Nanoporous structures with controlled geometrical characteristics
have demonstrated to be excellent materials for developing such optical devices as
resonators and microcavities [1-3]. So far, porous silicon (PSi) and nanoporous anodic alumina (NAA) have been successfully
used as platforms for fabricating those optical devices [4-6]. Concretely, the physical and chemical properties of NAA (e.g., biocompatibility,
thermal stability, environmental resistance, biodegradability, well-controlled geometry,
and so on) make it an excellent material for developing optical biosensors. One of
the foremost properties of NAA is that the pore geometry (i.e., cylindrical geometry)
can be exquisitely controlled by means of the anodization parameters (i.e., acid electrolyte,
anodization voltage, and anodization time). This allows us to change the effective
medium by designing the pore geometry and, thus, to control the specular reflection
at will.

Under certain conditions, the Rspecular spectrum of NAA presents oscillations generated by the Fabry-Pérot effect [7]. The number, position, and intensity of these oscillations rely on the NAA thickness
(i.e., the pore length (Lp)) and its porosity (i.e., the pore diameter (dp)). Therefore, this property offers us an excellent opportunity for designing NAA
structures with tunable optical properties by modifying the pore geometry. So far,
several works have proposed a systematic and objective system for classifying certain
optical properties of some nanostructures (e.g., PSi colloids, PSi nanowires, PSi
particles, silica nanotubes, and so forth) [8-11]. Recently, we presented a barcode system based on the photoluminescence of NAA and
its use for detecting biological substances was demonstrated [12]. Herein, we put forward an innovative barcode system based on the specular reflection
of NAA. In this system, an exclusive barcode is related to each set of pore length-pore
diameter by means of its Rspecular spectrum. In this way, a wide range of unique barcodes can be generated in the UV-visible
region. This barcode system is a useful method for estimating qualitatively and quantitatively
the responsiveness of optical biosensors based on NAA to changes in the effective
medium generated by external biological substances.

Furthermore, the pore walls in NAA can be functionalized with many materials (e.g.,
metals, oxides, polymers, etc.) in an accurate manner by such techniques as atomic
layer deposition, dip coating, and layer-by-layer deposition. This spreads the use
of NAA towards more excellent applications as selective separators, optical biosensors,
and so forth.

Methods

NAA samples were fabricated by the two-step anodization process [13]. Before anodizing, commercial aluminum (Al) substrates were electropolished in a
mixture of ethanol (EtOH) and perchloric acid (HClO4) 4:1 (v:v) at 20 V and 5 °C. After this, the first anodization step was performed in an aqueous
solution of oxalic acid (H2C2O4) 0.3 M at 40 V and 6 °C for 20 h. Subsequently, the alumina film was selectively
dissolved by wet chemical etching in a mixture of phosphoric acid (H3PO4) 0.4 M and chromic acid (H2CrO7) 0.2 M at 70 °C. Then, the second anodization step was conducted under the same anodization
conditions as the first step. The anodization time during this step was adjusted in
order to modify the pore length (i.e., 60, 105, and 150 min). Finally, the pore diameter
was enlarged by a wet chemical etching in an aqueous solution of H3PO4 5 wt.% at 35 °C.

NAA samples were characterized by an environmental scanning electron microscope (ESEM
FEI Quanta 600, Hillsboro, Oregon, USA. The specular reflection measurements were
performed in a UV-visible spectrophotometer from Perkin Elmer (Waltham, MA, USA) with
a tungsten lamp used as excitation light source at room temperature and the incoming
direction of light was 15°. The standard image processing package (ImageJ, public
domain program developed at the RSB of the NIH, USA) was used to carry out the ESEM
image analysis [14].

Results and discussion

To study the effect of the pore geometry on the Rspecular oscillations, the pore length and its diameter were widely modified. First, the pore
length of three samples was set to three different values (i.e., 5.0, 8.7, and 12.4 μm).
Then, the pore diameter of four samples with the same pore length (i.e., 5 μm) was
set to four different values (i.e., 30, 41, 52, and 71 nm). So, a total of seven different
NAA samples were fabricated and analyzed. Notice that, although the samples R-OSC(1)
and R-OSC(5) have the same geometric characteristics, they are different samples.
Figure 1 shows a set of top view ESEM images of four NAA samples with different pore diameters
and the same length. The results obtained from this image analysis are summarized
in Table 1. Figure 2a shows the Rspecular spectra as a function of the pore diameter. At first glance, it is verified that
the wider the pore, the higher the reflection intensity. Furthermore, it is observed
that the number of peaks decreases as the pore diameter is enlarged. Another result
that is worth noting is that, as Figure 2b shows, the larger the pore the lower the reflection intensity, and the number of
peaks increases with the pore length.

The origin of these oscillations in the specular reflection spectrum of NAA is related
to a strong enhancement of reflection at these wavelengths corresponding to the optical
modes of the Fabry-Pérot cavity constituted by the system air-NAA-aluminum. As a result
(p) marked and narrow oscillations are generated in the Rspecular spectrum, the number, intensity, and position of which not only can be tuned by increasing
the pore length but also by modifying its diameter (i.e., by changing the effective
medium).

The barcode system that we propose is based on the Universal Product Code [15]. In this system, each bar position corresponds to the wavelength of each oscillation
in the Rspecular spectrum, and the higher the oscillation intensity the wider the bar in the barcode.
The width of each bar is referred to the intensity scale. It means that the maximum
line width would correspond to the 100% intensity in the Rspecular spectrum. The bar width is reduced proportionally as the oscillation intensity decreases.

So far, some optical nanostructures have been successfully used as a base for developing
similar barcode systems [8-12]. However, from the biotechnological point of view, the optical encoding procedure
based on NAA has some advantages over those systems:

(a) The accurate control over the pore geometry in NAA makes it possible to switch
the effective medium of the Fabry-Pérot cavity at will. Therefore, the Rspecular spectrum can be designed and tuned for multiple applications. For example, those
barcodes with a high number of bars are envisaged for developing optical biosensors
with a high sensitivity to small external changes. Likewise, barcodes with a low number
of bars are more suitable for developing optical biosensors with a high specificity
(e.g., for substances with reflection maxima at localized wavelengths).

(b) The Rspecular spectrum of NAA remains stable throughout. Hence, it is not necessary to passivate
NAA for avoiding position shifts and intensity changes of the oscillations in the
course of time.

(c) The cylindrical geometry of pores in NAA allows covering the pore surface with
functional materials in a controlled manner for a wide range of applications.

An example of how the Rspecular oscillations are converted into an exclusive barcode is shown in Figure 3.

Figure 3.Example of conversion of aRspecularspectrum into a barcode. (a) Rspecular spectrum of sample R-OSC(1). (b) Resulting barcode after conversion.

Conclusions

In this study, we have presented an exhaustive analysis about the structural tuning
of the Rspecular oscillations in nanoporous anodic alumina. We have demonstrated that it is possible
to control the Rspecular oscillations at will by designing the pore geometry (i.e., structural engineering
strategy). This is an excellent opportunity for producing structures with controlled
optical properties, what is dearly useful for developing smart optical biosensors.
Furthermore, we have proposed a barcode system based on the oscillations in the Rspecular spectrum of NAA. This system is expected to be used to develop and design optical
sensors in such research fields as biotechnology and medicine.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

The experiments presented in this work were designed by AS and LFM. The NAAMs were
fabricated by AS and VSB, characterized optically and microscopically by AS and MA.
PF assisted AS and VSB during the laboratory tasks. AS, VSB, MA, PF, JFB, JP, and
LFM analyzed and discussed the results obtained from the experiments. AS wrote the
manuscript, and the last version of this was revised by all the authors (AS, VSB,
MA, PF, JFB, JP, and LFM). All authors read and approved the final manuscript.

Acknowledgments

This work was supported by the Spanish Ministry of Science and Innovation (MICINN)
under grant no. TEC2009-09551, CONSOLIDER HOPE project CSD2007-00007 and AGAUR 2009
SGR 549.